Method for preparation of cellulose nanocrystals and nanofibrils

ABSTRACT

A method for producing nanocellulose is described herein. The method includes contacting a cellulosic material with an oxidizing agent, and a compound selected from the group consisting of an alkali metal bromide, an alkali metal iodide, an alkali metal fluoride, an alkali metal chloride, or a combination thereof, in an aqueous solution to provide an oxidized cellulose mixture. The nanocellulose prepared according to the method is also described.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH & DEVELOPMENT

This invention was made with government support under the Materials Research Science and Engineering Center on Polymers (DMR-0820506) awarded by the National Science Foundation, and under CMMI-1025020 awarded by the University of Massachusetts Center for Hierarchical Manufacturing (CHM). The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

Derived from cellulose, one of the most abundant natural products on the planet, nanocellulose, including cellulose nanocrystals (NC or nanowhiskers) and cellulose nanofibrils (CNF), has a great number of interesting properties. Besides renewability and biodegradability, nanocellulose has low density, high elastic modulus, low thermal expansion coefficient, and good barrier properties. Potential applications of nanocellulose include but are not limited to packaging coatings, paper fillers, absorbent products, drug delivery, flexible displays, pharmaceuticals, separation membranes, fibers and textiles, batteries, supercapacitors, and electroactive polymers. See, e.g., Chem. Soc. Rev., 2011, 40, 3941-3994.

In general, nanocellulose is produced through acid hydrolysis, using sulfuric acid, hydrochloric acid, phosphoric acid, or hydrobromic acid. See, e.g., Chemical Reviews, 2010, 110, 3479-3500; U.S. Patent Publication No. 2010/0272819A1; and U.S. Pat. No. 5,629,055. The acid hydrolysis methods generally require harsh conditions and involve a substantial amount of concentrated acid. Strong acid can corrode production equipment and post-processing brings about a large amount of acidic waste, which is not environmentally friendly. See, e.g., Chinese Patent No. 101759807. Furthermore, methods utilizing harsh acids are hydrolytic, resulting in decreased yields and decreased fibril length.

Cellulose nanofibrils have been generally prepared by intensive mechanical treatment or a combination of enzyme or chemical pre-treatment with mechanical treatment. See, e.g., U.S. Pat. Nos. 8,871,056; 8,778,134; and 8,287,692. Most of the reported nanofibrillated cellulose suffers from irreversible agglomeration, namely, hornification, and therefore it cannot be redispersed in water after drying. See, e.g., Cellulose, 2010, 17, 793-802.

Accordingly, there remains a continuing need in the art for an acid-free, cost-effective method for the preparation of redispersible cellulose nanocrystals and nanofibrils.

BRIEF SUMMARY OF EMBODIMENTS OF THE INVENTION

One embodiment is a method for producing nanocellulose, the method comprising contacting a cellulosic material with an oxidizing agent, and a compound selected from the group consisting of an alkali metal bromide, an alkali metal iodide, an alkali metal fluoride, an alkali metal chloride, or a combination thereof, in an aqueous solution to provide an oxidized cellulose mixture comprising an oxidized cellulose precipitate, and mechanically treating the oxidized cellulose precipitate to provide oxidized cellulose nanofibrils.

Another embodiment is a method for producing nanocellulose, the method comprising contacting a cellulosic material with an oxidizing agent, a compound selected from the group consisting of an a compound selected from the group consisting of an alkali metal bromide, an alkali metal iodide, an alkali metal fluoride, an alkali metal chloride, or a combination thereof, and a nitroxyl radical compound, in an aqueous solution to provide an oxidized cellulose mixture comprising oxidized cellulose nanocrystals having an average diameter of 1 to 10 nanometers and an average length of 50 to 500 nanometers, wherein the nitroxyl radical compound is of the formula

wherein R¹ and R² are independently at each occurrence a C₁₋₆ alkyl group.

Another embodiment is an oxidized nanocellulose prepared by the method.

These and other embodiments are described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The following Figures represent exemplary embodiments:

FIG. 1 shows atomic force microscopy (AFM) height images of cellulose nanofibrils at a concentration of 0.1 weight percent after spin coating at 4000 rpm (A) and at a concentration of 0.01 weight percent after spin coating at 500 rpm (B). The images each show an area of 5 micrometers (μm) by 5 μm.

FIG. 2 shows scanning electron microscope (SEM) images of cellulose nanofibrils. The scale bar in (A) is 4 μm and the scale bar in (B) is 1 μm

FIG. 3 shows atomic force microscopy (AFM) height images of the cellulose nanocrystals obtained. (A) shows an area of 5 μm by 5 μm, and (B) shows an area of 1 μm by 1 μm.

FIG. 4 shows a transmission electron micrograph (TEM) of the cellulose nanocrystals obtained according to the method disclosed herein.

FIG. 5 shows a photograph of the oxidized cellulose nanofibrils obtained after drying (A), and the same oxidized cellulose nanofibrils following addition of water (B).

DETAILED DESCRIPTION OF THE INVENTION

The present inventors have discovered a method to produce well-dispersed cellulose nanofibrils and nanocrystals. Advantageously, the method disclosed herein does not require the use of harsh acids (e.g., sulfuric acid, hydrochloric acid, phosphoric acid, hydrobromic acid, and the like). Furthermore, the nanofibrils and nanocrystals produced by the method are easily redispersible. Additionally, a method for producing cellulose nanofibrils is provided herein that advantageously excludes the use of a nitroxyl radical compound such as 2,2,6,6-tetramethyl-1-piperidine-N-oxy (TEMPO), providing a cost-effective route toward cellulose nanofibrils.

Accordingly, one aspect of the present disclosure is a method for producing nanocellulose. The method comprises contacting a cellulosic material with an oxidizing agent and an alkali metal halide, for example, an alkali metal halide selected from the group consisting of an alkali metal bromide, an alkali metal iodide, an alkali metal fluoride, an alkali metal chloride, or a combination thereof in an aqueous solution. In some embodiments, the alkali metal halide is selected from the group consisting of an alkali metal bromide, an alkali metal iodide, or a combination thereof. The cellulosic material can be a cellulose raw material of plant origin, and can include a cellulose pulp, a refined cellulose pulp, kraft or sulfite pulp derived from various woods, or powdery cellulose obtained by pulverizing the pulp by a high-pressure homogenizer or mill or the like. In some embodiments, the cellulosic material comprises cellulose pulp. Cellulose pulp can be obtained, for example, by suitable treatment of softwood or hardwood. Softwood is for example, spruce fir, pine tree and larch. Hardwood is, for example, birch, beech, ash, aspen, and eucalyptus. Cellulose pulp can also be obtained from recycled paper products, for example recycled paper pulp formed from mill broke, pre-consumer waste, post-consumer waste or mixed office waste, cotton, flax, and hemp. Other sources of cellulose pulp are annual plants like rice, bamboo, and bagasse. Still other sources of cellulose pulp can be cellulose derived from various organisms including algae and sea cucumbers. Cellulose pulp can be bleached or unbleached cellulose pulp. When used as the cellulosic material, the cellulose pulp preferably has a cellulose content of at least 60%, for example at least 65, 70, 75, 80, 85 or 90%. Preferably, the cellulose pulp has a cellulose content of at least 70%. In some embodiments, the cellulose pulp can be refined prior to the contacting with an oxidizing agent, and a compound selected from the group consisting of an alkali metal bromide, an alkali metal iodide, or a combination thereof, for example by mechanical treatment.

The cellulosic material can be present in the aqueous solution in an amount of greater than 0 to less than or equal to 10 weight percent, based on the total weight of the aqueous solution. Within this range, the cellulosic material can present in an amount of at least 0.1 weight percent, or at least 0.5 weight percent, based on the total weight of the aqueous solution. Also within this range, the cellulosic material can be present in an amount of less than or equal to 5, or less than or equal to 1, or less than or equal to 0.75, or less than or equal to 0.5 weight percent.

The oxidizing agent can generally be any oxidizing agent capable of promoting an oxidation reaction including halogens, hypohalites, hypohalogenous acids, halogenous acids, perhalogen acids, alkali metal salts thereof, halogen oxides, peroxides, and the like. In some embodiments, the oxidizing agent is a hypohalite or an alkali metal salt thereof, for example lithium hypochlorite, potassium hypochlorite, sodium hypochlorite, calcium hypochlorite, magnesium hypochlorite, strontium hypochlorite, ammonium hypochlorite, and corresponding hypobromites and hypoiodites, or a combination thereof. In an embodiment, the oxidizing agent comprises sodium hypochlorite. The amount of oxidizing agent to be used in the method can be an amount effective to promote an oxidation reaction. For example, the oxidizing agent can be used in an amount of 0.5 to 500 millimoles (mmol), preferably 0.5 to 50 mmol, more preferably 2.5 to 25 mmol per gram of the cellulosic material.

In addition to the oxidizing agent, the cellulosic material is contacted with a compound selected from the group consisting of an alkali metal bromide, an alkali metal iodide, an alkali metal fluoride, an alkali metal chloride, or a combination thereof. In some embodiments, the compound is selected from an alkali metal bromide, an alkali metal iodide, or a combination thereof. The alkali metal bromides or alkali metal iodides that can be used are capable of dissociating into ions in water. For example, alkali metal bromides or alkali metal iodides that can be particularly useful include lithium bromide, potassium bromide, sodium bromide, lithium iodide, potassium iodide, sodium iodide, calcium bromide, magnesium bromide, strontium bromide, calcium iodide, magnesium iodide, strontium iodide, and the like, or a combination thereof. The amount of the alkali metal bromides or alkali metal iodides can be an amount effective to promote an oxidation reaction. For example, the alkali metal bromide or alkali metal iodide can be used in an amount of 0.1 to 100 millimole (mmol), preferably 0.1 to 10 mmol, more preferably 0.5 to 5 mmol per gram of the cellulosic material. In some embodiments, the method includes contacting the cellulosic material with the oxidizing agent, and the alkali metal bromide. In some embodiments, the alkali metal bromide comprises sodium bromide.

Contacting the cellulosic material with the oxidizing agent, and the compound selected from the group consisting of an alkali metal bromide, an alkali metal iodide, an alkali metal fluoride, an alkali metal chloride, or a combination thereof, provides an oxidized cellulose mixture. In some embodiments, the oxidized cellulose mixture comprises surface-oxidized cellulose, wherein the surface of the oxidized cellulose includes carboxyl groups, aldehyde groups, ketone groups, or a combination thereof. The degree of oxidation can vary from, for example, 25 to 100%, for example from 80 to 100%. The oxidized cellulose mixture comprises an oxidized cellulose precipitate.

The method further comprises separating the oxidized cellulose precipitate from the oxidized cellulose mixture. Separating can be by, for example, centrifuging, decanting, filtering, and the like. In some embodiments, the separating comprises centrifuging.

The method further comprises mechanically treating the oxidized cellulose precipitate to provide cellulose nanofibrils, preferably surface-oxidized cellulose nanofibrils. Without wishing to be bound by theory, it is believed that the surface oxidation allows for the preparation of cellulose nanofibrils that do not significantly aggregate. For example, it is believed that the adhesion between oxidized cellulose nanofibrils is reduced, preventing the formation of strong inter-nanofibril bonds (e.g., hydrogen bonds). The cellulose nanofibrils generally have an average diameter of 1 to 10 nm, for example 1 to 7 nm, for example 2 to 6 nm, for example 3 to 5 nm. The cellulose nanofibrils further have an average length of 500 to 2500 nm, for example 500 to 2000 nm, for example 800 to 2000 nm, for example 1000 to 2000 nm. In some embodiments, the mechanically treating comprises sonicating, milling, grinding, homogenizing, or mixing (e.g., using a high shear mixer or a high-pressure homogenizer), or a combination thereof. In some embodiments, the mechanically treating comprises sonicating. Mechanically treating the precipitated oxidized cellulose can disintegrate the precipitate into individualized cellulose nanofibrils.

In a specific embodiment, the method for producing nanocellulose comprises contacting a cellulosic material comprising cellulose pulp with an oxidizing agent comprising sodium hypochlorite, and an alkali metal bromide comprising sodium bromide, in an aqueous solution having a pH of 10 to 12 to provide an oxidized cellulose mixture comprising oxidized cellulose nanofibrils having an average diameter of 3 to 5 nanometers and an average length of 500 to 2500 nanometers. In some embodiments, the method excludes a nitroxyl radical compound. For example, contacting the cellulosic material with a nitroxyl radical compound is excluded from the method.

Another aspect of the present disclosure is a method for producing nanocellulose comprising contacting a cellulosic material with an oxidizing agent, a compound selected from the group consisting of an alkali metal bromide, an alkali metal iodide, an alkali metal fluoride, an alkali metal chloride, or a combination thereof, and a nitroxyl radical compound in an aqueous solution. The cellulosic material can be as described above. In some embodiments, the cellulose pulp can be refined prior to the contacting with an oxidizing agent, a compound selected from the group consisting of an alkali metal bromide, an alkali metal iodide, or a combination thereof, and a nitroxyl radical compound, for example by mechanical treatment.

The cellulosic material can be present in the aqueous solution in an amount of greater than 0 to 10 weight percent, based on the total weight of the aqueous solution. Within this range, the cellulosic material can present in an amount of at least 0.1 weight percent, or at least 0.5 weight percent, based on the total weight of the aqueous solution. Also within this range, the cellulosic material can be present in an amount of less than or equal to 5, or less than or equal to 1, or less than or equal to 0.75, or less than or equal to 0.5 weight percent.

The oxidizing agent can generally be any oxidizing agent capable of promoting an oxidation reaction including halogens, hypohalites, hypohalogenous acids, halogenous acids, perhalogen acids, alkali metal salts thereof, halogen oxides, peroxides, and the like, as described above. In an embodiment, the oxidizing agent comprises sodium hypochlorite. The amount of oxidizing agent to be used in the method can be an amount effective to promote an oxidation reaction. For example, the oxidizing agent can be used in an amount of 0.5 to 500 millimole (mmol), preferably 0.5 to 50 mmol, more preferably 2.5 to 25 mmol per gram of the cellulosic material.

In addition to the oxidizing agent, the cellulosic material is contacted with a compound selected from the group consisting of an alkali metal bromide, an alkali metal iodide, an alkali metal fluoride, an alkali metal chloride, or a combination thereof. In some embodiments, the compound can be selected from an alkali metal bromide, an alkali metal iodide, or a combination thereof. The alkali metal bromides or alkali metal iodides can be as described above. For example, alkali metal bromides or alkali metal iodides that can be particularly useful include lithium bromide, potassium bromide, sodium bromide, lithium iodide, potassium iodide, sodium iodide, calcium bromide, magnesium bromide, strontium bromide, calcium iodide, magnesium iodide, strontium iodide, and the like, or a combination thereof. The amount of the alkali metal bromides or alkali metal iodides can be an amount effective to promote an oxidation reaction. For example, the alkali metal bromide or alkali metal iodide can be used in an amount of 0.1 to 100 millimole (mmol), preferably 0.1 to 10 mmol, more preferably 0.5 to 5 mmol per gram of the cellulosic material. In some embodiments, the method includes contacting the cellulosic material with the oxidizing agent, the nitroxyl radical compound, and the alkali metal bromide, wherein the alkali metal bromide comprises sodium bromide.

In addition to the oxidizing agent and the compound selected from the group consisting of an alkali metal bromide, an alkali metal iodide, an alkali metal fluoride, an alkali metal chloride, or a combination thereof, the cellulosic material can optionally be contacted with a nitroxyl radical compound. The nitroxyl radical compound is of the formula

wherein R¹ and R² are independently at each occurrence a C₁₋₆ alkyl group. In some embodiments, R¹ and R² are independently at each occurrence a methyl group. In some embodiments, the nitroxyl radical compound is 2,2,6,6-tetramethyl-1-piperidine-N-oxy (TEMPO). TEMPO is a stable and water-soluble nitroxyl radical compound, and due to its steric hindrance, the primary hydroxyl groups on a cellulosic material are generally selectively converted to carboxylate groups. In some embodiments, the nitroxyl radical compound is used in a catalytic amount, for example 0.05 to 10 weight percent based on the dry weight of the cellulosic material, or 0.1 to 2.5 weight percent based on the dry weight of the cellulosic material.

According to the method disclosed herein, contacting the cellulosic material with the oxidizing agent, the compound selected from the group consisting of an alkali metal bromide, an alkali metal iodide, an alkali metal fluoride, an alkali metal chloride, or a combination thereof, and the nitroxyl radical compound in an aqueous solution provides an oxidized cellulose mixture. In some embodiments, the oxidized cellulose mixture comprises surface-oxidized cellulose, wherein the surface of the oxidized cellulose includes carboxyl groups, aldehyde groups, ketone groups, or a combination thereof. The degree of oxidation can vary from, for example, 25 to 100%, for example from 80 to 100%. The oxidized cellulose mixture comprises oxidized cellulose nanocrystals, preferably surface-oxidized cellulose nanocrystals. Without wishing to be bound by theory, it is believed that the surface oxidation allows for the preparation of cellulose nanocrystals that do not significantly aggregate. For example, it is believed that the adhesion between oxidized cellulose nanocrystals is reduced, preventing the formation of strong inter-nanocrystal bonds (e.g., hydrogen bonds). The cellulose nanocrystals generally have an average diameter of 1 to 10 nanometers (nm), for example 1 to 7 nm, for example 2 to 6 nm, for example 3 to 5 nm. The cellulose nanocrystals further have an average length of 50 to 500 nm, for example 75 to 450 nm, for example 90 to 400 nm, for example 100 to 400 nm.

In some embodiments, the method further comprises isolating the cellulose nanocrystals. Isolating the cellulose nanocrystals can be by, for example, centrifuging the aqueous solution comprising the oxidized cellulose mixture to provide an aqueous dispersion comprising the cellulose nanocrystals, and an oxidized cellulose precipitate. In some embodiments, the isolating can further include separating the aqueous dispersion comprising the cellulose nanocrystals from the precipitate, for example by decanting, filtering, and the like. Following isolation, the cellulose nanocrystals can be precipitated into a suitable solvent (e.g., acetone, tetrahydrofuran, ethyl acetate, or a combination thereof, washed, and dried. The dried cellulose nanocrystals are advantageously redispersible in an aqueous solution.

In some embodiments, subsequent to centrifuging the oxidized cellulose mixture, the method further comprises mechanically treating the oxidized cellulose precipitate to provide cellulose nanofibrils, preferably surface-oxidized cellulose nanofibrils. Without wishing to be bound by theory, it is believed that the surface oxidation allows for the preparation of cellulose nanofibrils that do not significantly aggregate. For example, it is believed that the adhesion between oxidized cellulose nanofibrils is reduced, preventing the formation of strong inter-nanofibril bonds (e.g., hydrogen bonds). The cellulose nanofibrils generally have an average diameter of 1 to 10 nm, for example 1 to 7 nm, for example 2 to 6 nm, for example 3 to 5 nm. The cellulose nanofibrils further have an average length of 500 to 2500 nm, for example 500 to 2000 nm, for example 800 to 2000 nm, for example 1000 to 2000 nm. In some embodiments, the mechanically treating comprises sonicating, milling, grinding, homogenizing, or mixing (e.g., using a high shear mixer or a high-pressure homogenizer), or a combination thereof. In some embodiments, the mechanically treating comprises sonicating. Mechanically treating the precipitated oxidized cellulose can disintegrate the precipitate into individualized cellulose nanofibrils. Accordingly, in an advantageous aspect of the present disclosure, the method described herein can provide the simultaneous synthesis of cellulose nanocrystals and cellulose nanofibrils.

In a specific embodiment, the method for producing nanocellulose comprises contacting a cellulosic material comprising cellulose pulp with an oxidizing agent comprising sodium hypochlorite, an alkali metal bromide comprising sodium bromide, and a nitroxyl radical compound having the formula

in an aqueous solution having a pH of 10 to 12 to provide an oxidized cellulose mixture comprising oxidized cellulose nanocrystals having an average diameter of 3 to 5 nanometers and an average length of 100 to 400 nanometers.

The contacting of the methods disclosed herein is carried out in an aqueous solution. The aqueous solution comprises water. In some embodiments, the aqueous solution further comprises a pH adjusting compound in an amount effective to provide a pH of 9 to 12, preferably 10 to 11. When present, the pH adjusting compound can be a basic compound, for example an alkali metal hydroxide, or an acidic compound, for example hydrochloric acid, and the like, or a combination thereof. In some embodiments, the pH adjusting compound comprises sodium hydroxide, potassium hydroxide, or a combination thereof, preferably sodium hydroxide. In some embodiments, the pH adjusting compound excludes an acidic compound. In some embodiments, the contacting is at a temperature of greater than or equal to 0 to 50° C., for example 10 to 30° C., for example 15 to 25° C.

In some embodiments, the combined yield of the cellulose nanocrystals and the cellulose nanofibrils (herein collectively referred to as “nanocellulose”) can be greater than or equal to 75%, for example greater than or equal to 80%, for example greater than or equal to 85%, for example greater than or equal to 85%, for example greater than or equal to 87%. The ratio of the cellulose nanocrystals to the cellulose nanofibrils can be controlled by adjusting the concentration of cellulosic material in the aqueous solution. For example, an aqueous solution having greater than 0.5 weight percent cellulosic material, for example greater than or equal to 1 weight percent cellulosic material, the weight ratio of cellulose nanocrystals to cellulose nanofibrils can be 1:10 to 1:2, for example 1:10 to 1:4. For an aqueous solution having less than or equal to 0.5 weight percent cellulosic material, the weight ratio of cellulose nanocrystals to cellulose nanofibrils can be 1:1 to 5:1, for example 1:1 to 75:1.

In some embodiments, the methods disclosed herein advantageously exclude the use of an acid in the treatment of the cellulosic material to provide the nanocellulose (i.e., no acid is required in the above-described method). Acids that can be excluded from the method include, for example, sulfuric acid, hydrochloric acid, phosphoric acid, hydrobromic acid, and the like. Thus, the method disclosed herein is an “acid-free” method for producing nanocellulose. In some embodiments, the method excludes any pre-treatment of the cellulosic material, for example by stirring or other mechanical treatment, a chemical treatment, an enzymatic treatment, or a combination thereof. As used herein, “pre-treatment” refers to any treatment conducted prior to contacting the cellulosic material with the oxidizing agent, the compound selected from the group consisting of an alkali metal bromide, an alkali metal iodide, or a combination thereof, and the nitroxyl radical compound in an aqueous solution. In some embodiments, the method includes a single oxidation step. In some embodiments, the method excludes a reduction step, for example the method excludes contacting the oxidized cellulose mixture with a reducing agent.

An example of each of the above-described methods is provided in the working examples below.

Another aspect of the present disclosure includes is a nanocellulose prepared according to the method disclose herein. The nanocellulose can be an oxidized cellulose nanofibril, an oxidized cellulose nanocrystal, or a combination thereof, wherein the oxidation can refer to surface oxidation. The oxidized nanocellulose comprises surface carboxylate groups. In some embodiments, the oxidized nanocellulose excludes surface sulfate groups. In some embodiments, the cellulose nanocrystals and nanofibrils prepared by the above-described method are single nanocrystals or single nanofibrils, for example, the nanocrystals and nanofibrils are not significantly aggregated. Advantageously, the oxidized cellulose nanofibrils and oxidized cellulose nanocrystals can be easily redispersed in an aqueous solution after drying to provide substantially transparent solutions.

The nanocellulose prepared by the method described herein can be useful for various applications, including packaging, coatings, paper filler, absorbent products, drug delivery, flexible displays, pharmaceuticals, separation membranes, fibers and textiles, batteries, reinforced composite materials, and supercapacitors. In particular, the nanocellulose can be useful for the preparation of light-weight, reinforced composite materials, transparent packaging films, and as fillers to enhance the mechanical properties of paper products.

The method disclosed herein can produce well-dispersed cellulose nanofibrils and nanocrystals without requiring the use of harsh acids. Furthermore, the nanofibrils and nanocrystals produced by the method are easily redispersible. Accordingly, a substantial improvement in the preparation of cellulose nanofibrils and nanocrystals is provided.

The invention is further illustrated by the following non-limiting examples.

EXAMPLES

Cellulose pulp was first treated with mechanical process, using either grinding, homogenization, microfluidization or high intensity ultrasonication, to prepare microfibrillated cellulose. Microfibrillated cellulose was then oxidized with solution of sodium hypochlorite, sodium bromide and, optionally, (2,2,6,6-tetramethylpiperidin-1-yl)oxy (TEMPO) or its derivatives at room temperature. Sodium hydroxide was added to adjust the pH to about 10-12. The mixture was vigorously stirred for a specific time (e.g., 20 hours), then centrifuged (e.g., at 4000 rpm for 15 minutes). The transparent supernatant was collected as a cellulose nanocrystal colloidal solution, which was then precipitated in acetone and washed with ethanol or methanol to remove residual salts and TEMPO, followed by drying in vacuum oven. The obtained cellulose nanocrystals generally have a diameter of about 3 to 5 nanometers (nm) and length of about 100 to 400 nm.

The precipitate isolated after centrifugation described above was washed and further centrifuged to remove residual salts and TEMPO, and then mechanically disintegrated in deionized water using ultrasonication, homogenization, or grinding. After mechanical treatment, a translucent aqueous mixture was acquired. The mixture was further centrifuged and the supernatant was collected as clear, transparent suspension of cellulose nanofibrils in water. The suspension was dried using rotary evaporation and porous, sponge-like cellulose nanofibrils were obtained. The isolated cellulose nanofibrils generally have a diameter of approximately 3 to 5 nm and length around of 1-2 micrometers.

By controlling the ratio of TEMPO and sodium hypochlorite relative to cellulose pulp as well as the reaction time, the relative yield of cellulose nanocrystals and cellulose nanofibrils can be controlled. Increasing the concentration of TEMPO and reaction time increases the yield of nanocrystals and decreases the yield of nanofibrils. If microfibrillated cellulose is oxidized without using TEMPO, only cellulose nanofibrils are obtained and almost no cellulose nanocrystals are observed.

Both the cellulose nanocrystals and cellulose nanofibrils prepared using the method described herein are redispersible in water using ultrasonication.

Experimental details follow.

Example 1

Preparation of Oxidized Cellulose Nanofibrils Without TEMPO

A solution of mechanically treated cellulose pulp (37.5 grams, 2.67% solid content) was dispersed in water (62.5 milliliters) containing sodium bromide (0.1 grams). A solution of sodium hypochlorite (2.1 grams, active chlorine 10-15%) was added to the suspension. Sodium hydroxide (5.5 grams, 0.5 M in water) was introduced to the mixture to adjust pH to about 12. The reaction mixture was stirred for 20 hours at room temperature, then centrifuged at 4000 rpm for 15 minutes. The supernatant was discarded, and the precipitate was redispersed in water and centrifuged to remove the remaining reagents. The redispersion and centrifugation was repeated three times until the pH of supernatant is close to DI water (about 6). After centrifuging, the precipitate was isolated, dispersed in 80 milliliters of water, and sonicated for 50 minutes to disintegrate the nanocellulose into single nanofibrils. The nanofibril solution was further centrifuged at 4000 rpm to remove the undisintegrated cellulose and the clear supernatant was collected as the product. The yield of the cellulose nanofibrils was estimated as 90.7% based on dry weight of cellulose nanofibrils relative to starting cellulose materials. The isolated cellulose nanofibrils prepared by this method were observed to have a diameter of about 3-5 nanometers and a length of about 600-1000 nanometers.

Atomic force microscopy (AFM) height images obtained using a DI Dimension-3000 atomic force microscope are shown in FIG. 1. The images show an area of 5 μm×5 μm. Scanning electron miscroscope (SEM) images obtained using a FEI Magellan 400 XHR-SEM are shown in FIG. 2. The scale bar in FIG. 2A is 4 μm and the scale bar in FIG. 2B is 1 μm.

Example 2

Preparation of Oxidized Nanocellulose With TEMPO

A solution of mechanically treated cellulose pulp (37.5 grams, 2.67% solid content) was dispersed in water (62.5 milliliters) containing sodium bromide (0.1 grams) and 2,2,6,6-tetramethyl-1-piperidine-N-oxy (TEMPO, 0.016 grams). A solution of sodium hypochlorite (2.1 grams, active chlorine 10-15%) was added to the suspension. Sodium hydroxide (5.5 grams, 0.5 M in water) was introduced to the mixture to adjust pH to 12. The reaction mixture was stirred for 20 hours at room temperature, then centrifuged at 4000 rpm for 15 minutes.

A cellulose nanocrystal colloidal solution was isolated as the transparent supernatant following centrifugation. The colloidal solution was precipitated in acetone and washed with ethanol or methanol three times to remove residual salts and TEMPO. The precipitated cellulose nanocrystals were dried at room temperature under vacuum for 24 hours. The cellulose nanocrystals were observed to have a diameter of about 3-5 nanometers and a length of about 100-400 nanometers.

The precipitate isolated following the initial centrifugation was further washed with deionized water and centrifuged the remove residual salts and TEMPO. This washing process was repeated three times. The precipitate was then dispersed in 80 milliliters of water, and sonicated for 50 minutes to disintegrate the nanocellulose into single nanofibrils. A translucent aqueous mixture was obtained following sonication. The mixture was centrifuged at 4000 rpm for 15 minutes, and the cellulose nanofibrils in water were collected as the transparent supernatant. The suspension was then lyophilized to obtain a porous, sponge-like material comprising the cellulose nanofibrils.

The relative ratio of cellulose nanocrystals to cellulose nanofibrils can be controlled by varying the concentration of cellulose in the starting reaction mixture, while keeping the other reagents (sodium hypochlorite, TEMPO, and sodium hydroxide) at the same concentration. For a reaction mixture having 1 weight percent cellulose, the yield of the cellulose nanocrystals was 12.7% and the yield of the cellulose nanofibrils was 75.2%. For a reaction mixture having 0.5 weight percent cellulose, the yield of the cellulose nanocrystals was 64.1% and the yield of the cellulose nanofibrils was 25.6%.

Atomic force microscopy (AFM) height images of the cellulose nanocrystals obtained above are shown in FIG. 3. FIG. 3A shows an area of 5 μm by 5 μm, and FIG. 3B shows an area of 1 μm by 1 μm. A scanning electron micrograph (SEM) of the cellulose nanocrystals obtained is shown in FIG. 4. The scale bar is 4 μm. As can be seen by both the AFM and SEM images of FIGS. 3 and 4, the cellulose nanocrystals are well-dispersed, and do not exhibit significant aggregation behavior.

Both the cellulose nanocrystals and cellulose nanofibrils prepared using the method described above are redispersible in water, as shown in FIG. 5A and 5B. FIG. 5A shows the oxidized cellulose nanofibrils obtained after drying. FIG. 5B shows the same oxidized cellulose nanofibrils following addition of water and sonication. As seen in FIG. 5B, a translucent aqueous solution is obtained indicating the ease with which the dried nanofibrils can be redispersed to prepared aqueous solutions.

The invention includes at least the following embodiments.

Embodiment 1: A method for producing nanocellulose, the method comprising, contacting a cellulosic material with an oxidizing agent, and a compound selected from the group consisting of an alkali metal bromide, an alkali metal iodide, an alkali metal fluoride, an alkali metal chloride, or a combination thereof, in an aqueous solution to provide an oxidized cellulose mixture comprising an oxidized cellulose precipitate, and mechanically treating the oxidized cellulose precipitate to provide oxidized cellulose nanofibrils.

Embodiment 2: The method of embodiment 1, wherein the method excludes a nitroxyl radical compound.

Embodiment 3: The method of embodiment 1 or 2, wherein mechanically treating comprises sonicating.

Embodiment 4: The method of any of embodiments 1 to 3, wherein the oxidized cellulose nanofibrils have an average diameter of 1 to 10 nanometers and an average length of 500 to 2500 nanometers.

Embodiment 5: A method for producing nanocellulose, the method comprising, contacting a cellulosic material with an oxidizing agent, a compound selected from the group consisting of an alkali metal bromide, an alkali metal iodide, an alkali metal fluoride, an alkali metal chloride, or a combination thereof, and a nitroxyl radical compound, in an aqueous solution to provide an oxidized cellulose mixture comprising oxidized cellulose nanocrystals having an average diameter of 1 to 10 nanometers and an average length of 50 to 500 nanometers, wherein the nitroxyl radical compound is of the formula

-   -   wherein R¹ and R² are independently at each occurrence a C₁₋₆         alkyl group.

Embodiment 6: The method of embodiment 5, further comprising isolating the oxidized cellulose nanocrystals.

Embodiment 7: The method of embodiment 6, wherein isolating the oxidized cellulose nanocrystals comprises centrifuging the oxidized cellulose mixture to provide an aqueous dispersion comprising the oxidized cellulose nanocrystals and an oxidized cellulose precipitate, and wherein the method further comprises mechanically treating the oxidized cellulose precipitate to provide oxidized cellulose nanofibrils.

Embodiment 8: The method of embodiment 7, wherein mechanically treating comprises sonicating.

Embodiment 9: The method of embodiment 7 or 8, wherein the oxidized cellulose nanofibrils have an average diameter of 1 to 10 nanometers and an average length of 500 to 2500 nanometers.

Embodiment 10: The method of any of embodiments 1 to 9, wherein the aqueous solution comprises a pH adjusting compound in an amount effective to provide a pH of 9 to 12.

Embodiment 11: The method of embodiment 10, wherein the pH adjusting compound is sodium hydroxide.

Embodiment 12: The method of any of embodiments 5 to 11, wherein each occurrence of R¹ and R² are methyl.

Embodiment 13: The method of any of embodiments 1 to 12, wherein the oxidizing agent is a hypohalite or an alkali metal salt thereof

Embodiment 14: The method of any of embodiments 1 to 13, wherein the oxidizing agent comprises sodium hypochlorite.

Embodiment 15: The method of any of embodiments 1 to 14, wherein the cellulosic material is contacted with the oxidizing agent, the alkali metal bromide wherein the alkali metal bromide is sodium bromide, and optionally the nitroxyl radical compound.

Embodiment 16: The method of any of embodiments 1 to 15, wherein the cellulosic material is cellulose pulp.

Embodiment 17: The method of embodiment 1, wherein the cellulosic material comprises cellulose pulp; the oxidizing agent comprises sodium hypochlorite; the cellulosic material is contacted with the oxidizing agent and the alkali metal bromide; the alkali metal bromide comprises sodium bromide; the aqueous solution has a pH of 10 to 12; and contacting the cellulosic material with a nitroxyl radical compound is excluded from the method.

Embodiment 18: The method of embodiment 5, wherein the cellulosic material comprises cellulose pulp; the oxidizing agent comprises sodium hypochlorite; the cellulosic material is contacted with the oxidizing agent, the nitroxyl radical compound, and the alkali metal bromide; the alkali metal bromide comprises sodium bromide; the nitroxyl radical compound has the formula

and

the aqueous solution has a pH of 10 to 12.

Embodiment 19: An oxidized nanocellulose prepared by the method of any of embodiments 1 to 18.

Embodiment 20: The oxidized nanocellulose of embodiment 19, wherein the oxidized nanocellulose is an oxidized cellulose nanofibril, an oxidized cellulose nanocrystal, or a combination thereof

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.

All cited patents, patent applications, and other references are incorporated herein by reference in their entirety, including priority U.S. Patent Application No. 62/240,036, filed Oct. 12, 2015. However, if a term in the present application contradicts or conflicts with a term in the incorporated reference, the term from the present application takes precedence over the conflicting term from the incorporated reference.

All ranges disclosed herein are inclusive of the endpoints, and the endpoints are independently combinable with each other. Each range disclosed herein constitutes a disclosure of any point or sub-range lying within the disclosed range.

The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. Further, it should further be noted that the terms “first,” “second,” and the like herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (e.g., it includes the degree of error associated with measurement of the particular quantity). 

1. A method for producing nanocellulose, the method comprising, contacting a cellulosic material with an oxidizing agent, and a compound selected from the group consisting of an alkali metal bromide, an alkali metal iodide, or a combination thereof, in an aqueous solution to provide an oxidized cellulose mixture comprising an oxidized cellulose precipitate, and mechanically treating the oxidized cellulose precipitate to provide oxidized cellulose nanofibrils.
 2. The method of claim 1, wherein the method excludes a nitroxyl radical compound.
 3. The method of claim 1, wherein mechanically treating comprises sonicating.
 4. The method of claim 1, wherein the oxidized cellulose nanofibrils have an average diameter of 1 to 10 nanometers and an average length of 500 to 2500 nanometers.
 5. A method for producing nanocellulose, the method comprising, contacting a cellulosic material with an oxidizing agent, a compound selected from the group consisting of an alkali metal bromide, an alkali metal iodide, or a combination thereof, and a nitroxyl radical compound, in an aqueous solution to provide an oxidized cellulose mixture comprising oxidized cellulose nanocrystals having an average diameter of 1 to 10 nanometers and an average length of 50 to 500 nanometers, wherein the nitroxyl radical compound is of the formula

wherein R¹ and R² are independently at each occurrence a C₁₋₆ alkyl group.
 6. The method of claim 5, further comprising isolating the oxidized cellulose nanocrystals.
 7. The method of claim 6, wherein isolating the oxidized cellulose nanocrystals comprises centrifuging the oxidized cellulose mixture to provide an aqueous dispersion comprising the oxidized cellulose nanocrystals and an oxidized cellulose precipitate, and wherein the method further comprises mechanically treating the oxidized cellulose precipitate to provide oxidized cellulose nanofibrils.
 8. The method of claim 7, wherein mechanically treating comprises sonicating.
 9. The method of claim 7, wherein the oxidized cellulose nanofibrils have an average diameter of 1 to 10 nanometers and an average length of 500 to 2500 nanometers.
 10. The method of claim 1, wherein the aqueous solution comprises a pH adjusting compound in an amount effective to provide a pH of 9 to
 12. 11. The method of claim 10, wherein the pH adjusting compound is sodium hydroxide.
 12. The method of claim 5, wherein each occurrence of R¹ and R² are methyl.
 13. The method of claim 1, wherein the oxidizing agent is a hypohalite or an alkali metal salt thereof.
 14. The method of claim 1, wherein the oxidizing agent comprises sodium hypochlorite.
 15. The method of claim 1, wherein the cellulosic material is contacted with the oxidizing agent, the alkali metal bromide wherein the alkali metal bromide is sodium bromide, and optionally the nitroxyl radical compound.
 16. The method of claim 1, wherein the cellulosic material is cellulose pulp.
 17. The method of claim 1, wherein the cellulosic material comprises cellulose pulp; the oxidizing agent comprises sodium hypochlorite; the cellulosic material is contacted with the oxidizing agent and the alkali metal bromide; the alkali metal bromide comprises sodium bromide; the aqueous solution has a pH of 10 to 12; and contacting the cellulosic material with a nitroxyl radical compound is excluded from the method.
 18. The method of claim 5, wherein the cellulosic material comprises cellulose pulp; the oxidizing agent comprises sodium hypochlorite; the cellulosic material is contacted with the oxidizing agent, the nitroxyl radical compound, and the alkali metal bromide; the alkali metal bromide comprises sodium bromide; the nitroxyl radical compound has the formula

and the aqueous solution has a pH of 10 to
 12. 19. An oxidized nanocellulose prepared by the method of claim
 1. 20. The oxidized nanocellulose of claim 19, wherein the oxidized nanocellulose is an oxidized cellulose nanofibril, an oxidized cellulose nanocrystal, or a combination thereof. 